The opponent — who has not been determined — holds an advantage over Rep. Gary Miller, R-Rancho Cucamonga, in next year’s election, according to Washington forecasters.

“I think Miller should be considered the underdog in the race at this point,” said Nathan Gonzales, deputy editor of The Rothenberg Report.

On Thursday The Cook Political Report changed its 31st District rating from a toss-up to leans Democratic. The alteration aligned Cook with The Rothenberg Report, which rated the district stretching from Upland to Redlands as leaning Democratic on June 14.

It is the only House race where the challenging party is favored by both nonpartisan ratings services.

“We don’t know which Democrat is going to emerge next June,” said David Wasserman, House Editor at Cook. “Whether it’s (Joe) Baca, (Pete) Aguilar or (Eloise) Gomez Reyes, any of them would have to be considered a serious threat, particularly Aguilar.”

Danny Tillman is also running as a Democrat, but has remained relatively quiet thus far.

The four Democrats and Miller will appear on the same ballot during the June 2014 primary, and potentially more could join the race. Under California’s jungle primary system, the top-two vote getters, regardless of party affiliation advance to the November election.

During its first year in effect, the open primary allowed Miller to gain the 31st District seat where 57 percent of voters chose Democratic President Barack Obama, according to the analysts.

“He owes his job to fluke election results in June 2012,” Wasserman said.

Four Democrats split the vote during that primary. Miller and fellow Republican Bob Dutton emerged from the primary and Miller won the general election.

“If he faced a Democrat, I think it would have been a different outcome,” Gonzales said.

But this cycle, the absence of another credible Republican candidate paves the way for a Democrat to advance to the November election.

This month’s partial government shutdown decreased Americans view of the Republican Party.

“It hands Democrats an issue to associate Miller with an unpopular Congress, even if he did vote to reopen the government,” Wasserman said.

Gonzales downplayed the impact the shutdown will have during next year’s election, but said, “Even if Republicans bounce back nationwide, this will be a tough district to win.”

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“It’s a Democratic district that Miller won because he faced a Republican in the general election,” Gonzales later added.

If Baca had chosen to run in the 31st last year — instead of the 35th where he lost to Democrat Gloria Negrete-McLeod — he would have beaten Miller and no Democrat would have run against him because he was an incumbent, according to Wasserman.

“Baca has only himself to blame for his current predicament,” he said.

Some of the Democrat candidates are being backed by various parts of the party.

Baca, the former congressman, has name recognition.

Gomez Reyes has gained the support of the women’s rights group Emily’s List, and the Democratic Congressional Campaign committee recruited Aguilar.

“So you have different parts of the Democratic machine lining up behind different candidates,” Gonzales said.